A Study of Gel Index and MRV Effects on SAE 5W-30 Pumpability

Today's engine oil formulators must continually balance performance in various low temperature rheological tests including Mini Rotary Viscosity (MRV), Scanning Brookfield Gel Index, Pour Point, etc. Frequently, optimization against one low temperature specification leads to sub-optimal performance in another, a direct consequence of significant differences in the cooling rates and shear rate/shear stress regimes employed in the tests. To better understand the implications of these differences in rheological properties, laboratory and vehicle low temperature tests were undertaken on a series of SAE 5W-30 oils with different combinations of gel index and MRV properties, but no direct correlation between the two. Full scale motored pumpability testing in pairs of modern 4- and 8- cylinder engines were conducted at ambient temperatures between -35 and -38°C (below the anticipated minimum start temperatures (MST's) of the vehicles). Pumpability testing showed acceptable performance in all the engines at temperature below the vehicles' MST. No correlation to the oils gel index was observed for either pressurization times or air-binding tendencies. However, pressurization times at the lowest temperatures did correspond to the MRV ranking of the oils. Significant pumpability differences were noted among the engines which may be associated with design and hardware differences. Analysis of the used lubricants after the motored pumpability testing also demonstrated some changes in gel index values, although MRV viscosities were comparable.